SPID#: 47 In order to determine whether there are age-related changes in the supragranular layer of the dentate gyrus of the rhesus monkey, we examined this layer in monkeys 4 to 35 years of age, spanning the entire range of adulthood of this species. Electron microscopic analyses of this region were conducted to determine whether there is an age-related change in the number of synapsing axon terminals, in the cross-sectional area of these terminals, or in the length of the postsynaptic density at the synaptic junction. In a subset of our monkeys (n = 6, ages 4 to 31 years), we compared three different approaches to the estimation of synaptic density (1) the conventional method, in which synapse numbers are expressed per unit area of the examined tissue section; (2) the empirical formula of Colonnier and Beaulieu for converting areal densities into number of synapses per unit volume; and (3) the disector method, a stereological approach to the estimation of number of synapses per unit volume that makes no assumption about the shape of the objects. The three methods were highly intercorrelated (r's = .89), and none of the methods revealed an age-related loss of synapses. In addition, the conventional method showed no age-related change in the number of axodendritic or axospinous synapses, the size of cross-sectional area of the synapsing terminals, or the length of the postsynaptic densities of synapses. Thus, since the synapses in the supragranular layer of the dentate gyrus do not change, one has to look somewhere else for structural changes that could be considered responsible for age-related cognitive decline and memory loss. Tissues of all monkeys were shared with other projects.